1. Field of the Invention
The invention relates generally to the deposition of a film on a substrate and more specifically to the use of sputtering to deposit films of weakly-conducting materials such as carbon or metallic oxides on substrates.
2. Description of the Prior Art
Sputtering is a much-used technique for depositing films on substrates. In sputtering, a source of material for the film called a target is placed in a vacuum chamber along with the substrates and the chamber is evacuated. After evacuation, the target is bombarded with ions. The ions knock material off the target, and the substrates are positioned in such fashion that the material which has been knocked off the target is deposited on the substrates. The most common source of ions is a plasma. To produce the plasma, the vacuum chamber is backfilled with an inert gas and a DC potential is applied between the target, which serves as a cathode, and an anode. The plasma forms in front of the target, and ions from the plasma bombard the target with the results just described. A common kind of sputtering apparatus is the magnetron. In the magnetron, a magnetic field is used to confine and enhance the plasma. The use of the magnetic field results in an increase in the number of ions generated for a given potential, and consequently a reduction in the potential required to create and maintain the plasma.
Sputtering apparatus which employs a plasma as an ion source is most effective when the target is made of material which conducts both heat and electricity well. Conduction of heat is important because the ion bombardment heats the target; if the target gets too hot, physical damage or changes in the composition of the target material may result. Conduction of electricity is important to prevent accumulation of surface charge on the target. The surface charge has the same polarity as the bombarding ions and therefore tends to repel the ions. If the target does not conduct electricity, radio frequency oscillations must be applied to the plasma, adding to the complexity of the sputtering apparatus and the sputtering process. Details concerning all of the above may be found in Rointain F. Bunshah et al., Deposition Technologies for Films and Coatings, Noyes Publications, Park Ridge, N.J., 1982, pp. 170-243.
The limitations of plasma sputtering with regard to materials which do not conduct heat or electricity well have become more important with the discovery of high temperature superconducting metallic oxide compounds. At ordinary temperatures, these compounds are weak conductors of heat and electricity like other metallic oxides, but when cooled to temperatures in the neighborhood of the boiling point of nitrogen (77 degrees K.), fine wires or thin films of the compounds become superconductors of electricity.
The usefulness of high-temperature superconductors in electronic devices depends on the development of inexpensive techniques for depositing them in thin films on substrates. The use of magnetron sputtering to produce such thin films has been made difficult by the weak electrical and heat conductivity of the high-temperature metallic oxide superconductors. These problems are exacerbated by the fact that bulk amounts of high temperature superconductors are made from a powder of the superconducting compound by compacting the powder into the desired shape and then sintering it so that the particles of the powder adhere to each other. The mechanical properties of the sintered powder are poor and the sintered powder is an even worse conductor of heat and electricity than thin films or wires of the superconducting compound. In consequence, the prior art has not been able to use large targets or high potentials to sputter superconducting compounds and has thus been able to sputter superconducting compounds on only a small scale. One example of such small-scale sputtering using a magnetron is found in Hoehler and Neeb, "Preparation of Thin YBa.sub.2 CUsub3O.sub.7-8 Layers on Various Substrates . . . by d.c. Magnetron Sputtering" , Journal of the Less-common Metals, 151 (1989) pp. 341-344.
What is needed if films of high-temperature superconducting materials are to be produced at low cost and high volume is a technique for magnetron sputtering of such materials which overcomes the problems caused by the poor conductivity of heat and electricity which is characteristic of the compounds and the exacerbation of these problems which result from the use of sintered targets. Such a technique is provided by the present invention.